In general, perforating guns are lowered into wellbores by a number of means, including electric line, or rigid or coiled tubing. The guns are supported from the surface or supported by a device downhole in proximity of the perforating gun. Utilizing surface suspension, perforating guns are run to correct depth via electric line, or rigid or coiled tubing and fired at a desired depth. The fired guns are then pulled to surface or occasionally released and dropped to the bottom of the wellbore. Downhole suspension of the perforating guns is accomplished by a number of means. Utilizing rigid or coiled tubing to run perforating guns enables a hookwall packer to anchor guns in a cased hole or inflatable packer to anchor guns in an open hole. Using electric line enables an electric line activated permanent packer to suspend a perforating gun at a desired depth.
The prior assemblies have several shortcomings. If using electric line to run and position a gun, gun length is limited by the length of the surface lubricator when perforating a wellbore underbalanced. When an electric line set packer is utilized to anchor a gun, a time consuming milling operation is required to release the packer.
If using downhole suspension to anchor the perforating guns, other disadvantages are encountered associated with coiled or rigid tubing. Using rigid tubing requires make up of the string at the surface and necessarily creates significant delays in positioning the gun in the wellbore for perforation. Use of rigid tubing also requires that the reservoir be stabilized prior to removal. This results in higher operational cost and excess reservoir damage which limits reservoir profitability. The use of coiled tubing may be restricted due to cost or available rig space considerations.
Those skilled in the art realize that the downhole perforating gun anchor may be located above, below or anywhere along the length of the perforating gun. When support for the perforating gun is downhole from the gun and problems ensue in the release of the gun, operations are necessarily made more complicated to try and remove the gun. Since support for the gun is downhole, if the gun becomes stuck and requires milling to be removed, the entire length of the gun has to be milled prior to the support system or slips being reached so that the assembly can drop to the bottom of the wellbore.
The apparatus and method of the present invention provide support for the gun uphole from the gun. Therefore, if problems develop in firing the gun, ordinary fishing tools can be used to remove it. If milling is required to remove the gun, substantially less milling is required prior to the release of the slips so that the remaining anchor assembly with the gun below can fall to the bottom of the wellbore.
In order to address the concerns about having to jar with electric line, the apparatus and method of the present invention have been developed to allow hydraulic set of the anchor for the perforating gun when the assembly is run in the hole with an electric line, or rigid or coiled tubing. A running tool for the anchor gun assembly has been designed to release primarily by hydraulic pressure in the wellbore with a secondary backup of mechanical release. Accordingly, the apparatus and method of the present invention can be used to hydraulically set an anchor in situations where the setdown weight may be too low for a setting mechanism relying on relative movement due to weight of the components.
The apparatus and method also provide for a way to release the anchor and drop the gun if for any reason it has failed to automatically release when shot. Means are provided which, in the preferred embodiment, are mechanical to allow overriding of the grip of the slips to release the gun and anchor assembly to the bottom of the wellbore. Alternatively, the anchor can be constructed so that it is not automatically released after the gun is shot. A retrieving tool can then recover the assembly of the gun and anchor from the wellbore.
A positive release of the slips using a pressure balanced piston whose movement sequentially undermine the cones supporting the slips in the engaged position is also provided to assure a smooth release from the casing or wellbore by the slips.